1. Field of the Invention
This invention relates generally to a seed structure in a film stack for re-setting the crystalline structure of an upper layer from the amorphous or crystalline structure of a lower layer, and more particularly to a perpendicular magnetic recording (PMR) disk that uses the seed structure for magnetically decoupling two magnetic layers.
2. Description of the Related Art
In current thin film technology it is often desired to fabricate high quality thin films with a desired functionality within a more complex film stack. It is often difficult to orient the crystallites within a specific film in the stack with the same crystallite direction to the surface normal when starting from a flat surface of a substrate which may be amorphous or crystalline. For example, in one type of PMR disk an exchange break layer (EBL) is required between the soft magnetic underlayer (SUL) and the magnetic recording layer (RL). The EBL should be as thin as possible to improve writing to the RL and must set the crystalline orientation of the RL while also acting to magnetically decouple the SUL and the RL. In another type of PMR disk that uses a “laminated” RL, a nonmagnetic spacer layer (SL) is required to magnetically decouple two RLs and to de-correlate the magnetic microstructure in the two RLs to obtain independent averaging of the signal from the two RLs to thereby increase the readback signal-to-noise ratio (SNR).
PMR disks with “bit-patterned media” (BPM) have been proposed to increase the data density. In BPM disks, the magnetizable material of the RL on the disk is patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. A problem associated with BPM is the relatively wide variation in the coercive field among the individual magnetic islands. This variation is characterized by a wide distribution of the switching field, i.e., the write field required to switch the magnetization of a magnetic island from one state to the other state. Ideally the switching field distribution (SFD) width would be zero, meaning that all the bits would switch at the same write field strength. A well-defined crystalline structure in the RL in the individual islands is required to obtain a narrow SFD width.
What is needed is a seed structure that re-sets the crystalline structure of an upper layer from the amorphous or crystalline structure of a lower layer and, when located between two magnetic layers, magnetically decouples the layers, de-correlates the magnetic microstructure of the two layers, and provides a template for the well-defined crystalline growth of the upper magnetic layer.